In recent years, synchronous induction motors have begun to be used in an electric hermetic compressor mounted in refrigerators, air-conditioners or the like to improve the system efficiency. A starter for the motor includes a PTC relay equipped with a built-in Positive Temperature Coefficient thermistor. Japanese Patent Unexamined Application No. 2002-300763 discloses an example of such a motor and a compressor.
Now, a conventional synchronous induction motor and an electric hermetic compressor using the motor are described with reference to the drawings. FIG. 11 shows a cross sectional view of the conventional electric hermetic compressor. FIG. 12 shows a cross sectional view of a rotor of the conventional synchronous induction motor. FIG. 13 shows a circuit diagram of the conventional synchronous induction motor.
Hermetic housing 6 encloses synchronous induction motor 1 and compression unit 5 driven by motor 1. Motor 1 starts as an induction motor and runs as a synchronous induction motor in sync with the supply voltage frequency at steady state running. Motor 1 has stator 2 and rotor 3. Stator 2 consists of main winding 11 and auxiliary winding 12 wound on a core (not shown) made of electric steel sheet laminations. Rotor 3 encloses permanent magnets 10 disposed in yoke 9 also made of electric steel sheet laminations, and has aluminum-made secondary conductors 4 disposed in a vicinity of the periphery of yoke 9.
Hermetic terminal 7 connects operating capacitor 15 to auxiliary winding 12. Starting capacitor 14 and PTC relay 8 that includes positive temperature coefficient thermistor 13 are connected in parallel with operating capacitor 15 and are connected in series with auxiliary winding 12.
FIG. 14 shows speed vs. torque curves of a synchronous induction motor, where the horizontal axis denotes rotation speed of motor 1 and the vertical axis denotes torque force.
Curve 87 shows an inherent torque characteristic of the induction motor. Resultant torque 81 is result of adding brake torque 88 generated by permanent magnets 10 to the torque characteristic shown by curve 87. Resultant torque 81 represents the output torque of motor 1. Torque 90 represents the starting torque of motor 1 and torque 83 represents the maximum torque of motor 1. Torque 84 represents an output torque at a synchronous speed, and motor 1 usually runs in a synchronous operation with loading under the maximum value of torque 84.
Next, the operation of motor 1 with the aforementioned configuration and an electric hermetic compressor using the same are described.
Upon energizing, starting current flows into main winding 11, auxiliary winding 12, thermistor 13, starting capacitor 14 and operating capacitor 15. When flowing the starting current, main winding 11 and auxiliary winding 12 establish a rotating magnetic field, which induces induction currents on secondary conductors 4, causing rotor 3 to generate its own magnetic field. With starting torque 90 obtained from the magnetic field, rotor 3 starts running and continues to accelerate the speed along with output torque curve 81. After approaching the synchronous speed, then the motor reaches a synchronous operation at the synchronous speed generating torque 84.
At the same time, current flowing into thermistor 13 generates self-heating in thermistor 13, causing thermistor 13 to increase in temperature and subsequently in resistance value rapidly. Consequently, current flowing into starting capacitor 14 is substantially cut off, and motor 1 continues running at the synchronous speed.
Next, rotor 3 drives compression unit 5 to carry out a known compressing operation.
However, a minute electric current continues to flow into thermistor 13 for the heating to keep the resistance in a high value during running of motor 1 with the aforementioned conventional configuration. To provide motor 1 with a required torque property, an amount of torque is needed as described above to compensate the torque offset by the brake torque generated in permanent magnets 10. For this reason, motor 1 needs a larger magnetic inductive torque compared with ordinary induction motors.
Typically, to increase the magnetic inductive torque, the number of windings of auxiliary winding 12 is increased to have a larger winding ratio. As a result, however, a higher voltage is induced in auxiliary winding 12 causing a higher voltage applied on PTC relay 8 connected to auxiliary winding 12 as well. Thermistor 13 is thus required to have a higher voltage resistance than ordinary induction motors to withstand the voltage.
To increase the voltage resistance of thermistor 13, thermistor 13 should have a larger volume. Such a configuration needs to increase the amount of heat emission required to maintain the resistance in a high value, causing PTC relay 8 to increase the power for self-consumption to a higher level of 3 to 4 watts, thereby causing motor 1 to decrease the system efficiency greatly. The system efficiency of the electric hermetic compressor having motor 1 with this configuration consequently decreases.
Additionally, the enlarged diameter of thermistor 13 increases the heat capacity of thermistor 13, causing difficulty in fast cooling. Namely, a longer time is required to cool thermistor 13 to a temperature ready for re-starting, resulting in a poor re-starting property for the electric hermetic compressor.